1. Field of the Art
This invention relates to a polarization conversion element to be used on various optical appliances, and to an optical system containing such a polarization conversion element.
2. Prior Art
A beam splitter is used on various optical appliances as an optical element for splitting an incident light flux into two or more light paths. A certain type of beam splitter functions to split incident light into a number of light paths on the basis of diffraction of light, while another type of beam splitter functions to split incident light by transmission and reflection of light. An optical element which splits light by transmission and reflection of light is generally known as a half mirror. In this regard, half mirrors are not necessary made to split light equally into a number of light paths. Certain half mirrors are adapted to split light at an arbitrary ratio.
A half mirror is fabricated by laminating an optical film on a transparent material like optical glass. The laminating optical film can be a semi-transmitting type which is adapted to reflect incident light at a predetermined rate while transmitting the remainder, or a polarizing type which is adapted to reflect one of two polarized components with perpendicularly intersecting planes of polarization while transmitting the other component. Therefore, optical elements which are called “a half mirror” in a broad sense include a semi-transmitting film type and a polarizing film type which should be called “a half mirror” and “a polarizing beam splitter” in a narrow sense, respectively. In splitting incident light by reflection and transmission, the half mirror in the narrow sense can reflect and transmit light at an arbitrary ratio by controlling the transmission rate through the semi-transmitting film. On the other hand, of two polarized components with rectangularly intersecting planes of polarization, certain polarizing beam splitters are arranged to have such properties as to transmit one polarized component 100% while reflecting the other polarized component 100%. In this regard, it is possible to hold the transmission rate of one polarized component and the reflection rate of the other polarized component at a value less than 100%. The former can be classified as “complete beam splitter” and the latter can be classified as “incomplete beam splitter.”
Polarized components of incident light, which is cast on a half mirror for splitting a light path, include linearly polarized light, circularly polarized light and elliptically polarized light. Even linearly polarized light may contain not only a component having a plane of polarization in perpendicularly intersecting relation relative to a certain plane of polarization but also components having a plane of polarization at an arbitrary angle relative to the certain plane of polarization. Further, elliptically polarized light includes a wide range of components from a component akin to circular polarization to a component of flat elliptic polarization which can be almost regard as linear polarization. Besides, the line of apsides lies in an arbitrary direction.
Gathering from the foregoing, normally a half mirror of the narrow sense is used as an optical element which transmits and reflect incident light in a controlled ratio. On the other hand, it has been the general practice to use a polarizing beam splitter particularly for splitting incident light having perpendicularly intersecting polarized components which are called p- and s-polarized components.
In the case of the half mirror, a beam splitter in a narrow sense, as a semi-transmitting film a metal film is laminated on a transparent member like optical glass. In order to control the transmission and reflection ratio, the metal film is formed by the use of metals such as silver (Ag), aluminum (Al), gold (Au) and chromium (Cr). Of these metals, silver and aluminum are low in wavelength dependency. That is to say, in order to split light of a broad wavelength range, it is necessary to use silver or aluminum. Comparing semi-transmitting films of the same transmission rate, it is preferable to employ a semi-transmitting film of silver rather than aluminum for the control of transmission rate, taking into account the effort which is required in making the aluminum film thinner. However, due to dependency on polarized components of incident light in addition to dependency on wavelength, it is difficult even for silver to split incident light strictly in a desired transmission and reflection ratio over a wide wavelength range. Above all, in the fields of optical pickups and other recently developed high precision optical devices like photo-computing devices, one faces difficulties in getting accurate light splitting characteristics by the use of a half mirror laminated with a silver semi-transmitting film.
In connection with a half mirror using a semi-transmitting film of silver, Japanese Laid-Open Patent Application 2001-13308 discloses a half mirror with a function of correcting variations in transmission-reflection ratio attributable to wavelength dependency and polarized components of incident light. According to this published patent literature, as a semi-transmitting film to be laminated on a half mirror (in a narrow sense), a metal film of silver is combined with a dielectric layer of low or medium refraction material and a dielectric layer of high refraction material to solve the problem of variations in transmission-reflection ratio due to wavelength dependency and polarized components.
Further, silver is problematic in weather resistance, namely, characteristics of a silver semi-transmitting film changes over a long period of time, with progress of corrosion which occurs under the influence of temperature and humidity. Besides, if incident light is of a wide range in wavelength, it may become difficult in some cases to prevent marked variations in transmission-reflection ratio and to guarantee sufficient reliability. In the case of the half mirror of the above-mentioned published patent literature, three films of different materials, i.e., a metal film and two dielectric films, have to be formed to stabilize the transmission-reflection ratio. In addition, considering the necessity for laminating a protective film layer on the metal film for the improvement of the weather resistance, it is likely that the fabrication process will become difficult and result in an expensive optical element.
Polarizing beam splitters, i.e., another type of half mirror in a broad sense, are stabilized in optical characteristics because no metallic films are used as optical films, and can maintain a predetermined transmission-reflection ratio accurately over a wide wavelength range by providing a suitable number of polarizing films of an adequate thickness. However, it is difficult to split incident light in a desired ratio without controlling polarized components in the incident light flux.
In splitting incident light in a controlled ratio, optical characteristics of the above-mentioned polarizing beam splitters do not necessarily require that the incident light be composed of two perpendicularly intersecting linearly polarized components. Namely, according to optical characteristics of polarizing beam splitters, it is possible to accurately control the transmission-reflection ratio for two perpendicularly intersecting polarized components of the same energy profile even if the two components are not linearly polarized in a strict sense. That is, in case two perpendicularly intersecting components of incident light on a polarizing beam splitter are of the same energy profile, the polarizing beam splitter can split the incident light substantially in the same way as incident light consisting of two perpendicularly intersecting linearly polarized components. Accordingly, if it becomes possible for a polarizing beam splitter to convert incident light into perpendicularly intersecting polarized components with the same energy profiles in an initiatory or introductory stage, the polarizing beam splitter can find a wide range of applications because it is reliable in weather resistance and stable in optical characteristics as a light splitting means.